The multiples of 7 form an Arithmetic Progression with common difference 7. 200 ÷ 7 = 28.57... and 400 ÷ 7 = 57.14... ∴ The first multiple of 7 after 200 will be 7 × 29 = 203 and the last multiple of 7 before 400 will be 7 × 57 = 399. Let's say that 399 is the n‌th term of the AP. ∴ 399 = 203 + (n - 1)7 ⇒ 7n - 7 = 196 ⇒ 7n = 203 ⇒ n = 29 This means, there are 29 terms from 203 to 399 with a common difference of 7. The sum of these numbers will be: S29=(